Recently, portable electronic devices such as portable computers, portable telephones, camcorders, and the like have been continuously developed towards smaller and lighter devices. In line with such trend, secondary batteries used as a power source of these electronic devices require high capacity, small size, and light weight. Thereamong, research on lithium ion secondary batteries is actively conducted due to excellent characteristics thereof such as high voltage, long lifespan, high energy density, and the like and such lithium ion secondary batteries are produced and commercially available.
As a conventional cathode active material of a lithium secondary battery, a lithium-containing cobalt oxide (LiCoO2) is mainly used. In addition, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, and the like, and lithium-containing nickel oxides such as LiNiO2 is also considered.
In this regard, charging and discharging are performed while lithium ions are intercalated into and deintercalated from a cathode active material. Although there are differences in theoretical capacities of batteries according to kinds of cathode active materials, in most cases, charge and discharge capacities are deteriorated as cycles proceed. Such phenomenon is mainly attributed to non-functioning of a cathode active material due to change in volume of a cathode occurring as charging and discharging of a battery proceed, such as separation of cathode active material components or separation between the cathode active material and a current collector, elution of metals, and the like.
In addition, polyvinylidene fluoride (PVdF), which is a polymer resin, is widely used as a binder. However, when moisture permeates an electrode, PVdF forms HF and, accordingly, a metal layer of a cathode is decomposed, which results in deteriorated battery performance.
To address this problem, a method of using, as water-based binders, rubber-based latexes such as styrene-butadiene rubber (SBR) is taken into consideration. SBR is environmentally friendly and enhances capacity and initial charge and discharge efficiency of secondary batteries when used in a reduced amount. Even in this case, however, while adhesion sustainability of SBR is enhanced due to elasticity of rubber, adhesion effects are low. Thus, SBR is not suitable for high-capacity active materials, which require an electrode with high adhesion because volumetric expansion is large when metal-based active materials are subjected to charging and discharging and thus use thereof is restricted.
To simultaneously address these problems, the related art proposes a method of coating a surface of a cathode active material with a predetermined material or a technology of surface-treating a cathode active material. However, cathode active materials that can fundamentally address the problems described above have not been developed yet.
Therefore, there is an urgent need to develop an electrode material that may enhance battery performance such that separation of cathode active material components or separation between a cathode active material and a current collector is prevented, volume expansion of a cathode active material occurring during repeated charging and discharging is controlled, adhesion is enhanced, and side reaction in the vicinity of a cathode is suppressed.